GC54B-01 INVITED
Permanent El Nino Conditions in the Early Pliocene, the Poleward Heat Transport Paradox, and Contemporary Global Warming.
Proxy temperature records show that in the early Pliocene, approximately 3 to 5 million years ago, the tropics were characterized by permanently warm El Nino-like conditions. The equatorial Pacific was as warm as in the east as it is in the west today, and the zonal SST gradient along the equator was significantly reduced or absent. Concurrently, major coastal upwelling regions were up to 10 degrees C warmer than they are today. The globally averaged temperatures of the Earth's surface were also substantially higher. This climate state persisted even though the external factors that control climate were essentially the same as at present and the Earth was experiencing greenhouse conditions similar to today's, with the concentration of CO2 in the atmosphere comparable to present day values. Thus far, there is no satisfactory explanation for the climate state of the Pliocene, especially for the climate conditions in the tropics and subtropics. State-of-the-art climate models fail to reproduce a permanent El Nino even when forced by CO2 concentrations many times larger than those estimated for the early Pliocene. Predicting the impact on the tropics of global warming caused by anthropogenic factors also remains a serious challenge for climate scientists. Coupled general circulation models yield a wide range of possible scenarios for the region, but many suggest a slightly higher likelihood of an El Nino-like state in global warming. Efforts to predict future global warming should benefit enormously from a better understanding of the state of permanent El Nino which imposes a strong dynamical constraint on both oceanic and atmospheric circulations. Modeling permanent El Nino with atmospheric and oceanic GCMs reveals a poleward heat transport paradox: Calculations with ocean-only models suggest that a permanent El Nino should correspond to a reduced poleward heat transport by the ocean. This is related to a deeper thermocline in the eastern equatorial Pacific, which leads to a smaller ocean heat intake in the equatorial region. However, calculations with atmospheric GCMs suggest a weaker poleward heat transport by the atmosphere and, consequently, a strengthening of the ocean heat transport. This contradiction implies that there should be an additional mechanism for the poleward heat transport, which is absent or under-resolved in the current generation of general circulation models. This factor explains why climate models cannot replicate a permanent El Nino, and also questions whether climate GCMs can model adequately the impacts of global warming.
GC54B-02
Insights Into Deglacial Through Holocene Climate Variability At The Peru-Chile Margin From Very High Sedimentation Rate Marine Cores
The Peru-Chile upwelling system is situated at the epicenter of the modern ENSO System. The high settling flux of organic materials and poor ventilation of subsurface waters makes the Peru upwelling system one of the world's three major oxygen minimum/denitrification zones (Codispoti and Christensen, 1985). Extremely high sedimentation rates and permanent hypoxic/anoxic subsurface waters create excellent conditions for the preservation of organic matter. Despite the significance of this region in regards to paleoceanography and paleoclimatology, relatively little work has been done to characterize past Peruvian climate because carbonate dissolution hinders the use of conventional paleoclimate methods and hiatuses frequently interrupt the record. However, using nitrogen isotopes and alkenone paleothermometry on multiple sediment cores from the Margin we have managed to overcome many of these challenges to create a nearly continuous SST (Uk`37), productivity (C37total), biogenic opal and denitrification (δN15) record from the LGM through the late Holocene. Remarkably, recent work has revealed an annually laminated core, which spans from 1.4-8.0ka uninterrupted, providing a unique window into Holocene climate variability. Modern-day upwelling induced climate at the Peru-Chile margin is characterized by cold temperatures (21.5°C) high productivity and strong denitrification, which has persisted since the mid Holocene (4ka). The mid Holocene also marks the beginning of a dramatic increase in seasonality and ENSO variability consistent with other tropical climate indicators. Climate variability in the Mid-early Holocene shows a distinctively different pattern from that of the late Holocene; unproductive warm temperatures persist through the early Holocene in what can be described as a permanent El Niño-like state. Early tropical warming occurred near 17ka along with an unprecedented increase in denitrification, which is decoupled from local productivity. Early onset of denitrification relative to deglaciation indicates a high latitude remote forcing that was rapidly translated to the tropics at the close the last glacial period.
GC54B-03
Abrupt changes of ENSO variability due to orbital and millennial-scale climate change
Paleo evidence from fossil corals and lake records suggests that ENSO is modulated on orbital timescales. The famous mid-Holocene ENSO suppression e.g. has been attributed to orbitally-induced background state and annual cycle changes. Using an accelerated orbitally-driven CGCM simulation representing the period from 142,000 years B.P (before present) to 22,900 years A.P. (after present), the fundamental mechanisms are explored that lead to the generation of precessional cycles in the tropics. Due to the mean seasonal cycle of cloudiness in the off- equatorial regions, an annual mean precessional signal of temperatures is generated outside the equator. The resulting meridional SST gradient in the eastern equatorial Pacific modulates the annual mean meridional asymmetry and hence the strength of the equatorial annual cycle. In turn, changes of the equatorial annual cycle trigger abrupt changes of ENSO variability via frequency entrainment, resulting in an anti-correlation between annual cycle strength and ENSO amplitude on precessional timescales. We furthermore demonstrate that a similar mechanism operates on millennial timescales. Changes of the Atlantic Meridional Overturning Circulation (AMOC), lead to changes of the meridional SST gradient in the eastern tropical Pacific, and hence a modulation of the strength of the annual cycle and via nonlinear frequency entrainment also of ENSO. Analysis of a multimodel-ensemble of waterhosing experiments, recently conducted as part of CMIP-2, reveals that a collapse of the AMOC leads to the disappearance of the equatorial Pacific annual cycle and an intensification of ENSO variability. Both, on orbital and millennial timescales, meridional SST gradients play a crucial role in modulating ENSO variability. Whether this new paradigm is also applicable to future greenhouse warming experiments is still an open question.
GC54B-04 INVITED
North Pacific Gyres Oscillation: a tropical source of ocean climate and ecosystem change
Decadal fluctuations of salinity and key biological variables in the Northeast Pacific Ocean over the past 50 years have been well documented and are thought to be related to climate change. Dramatic fluctuations in salinity, nutrients, chlorophyll, zooplankton biomass, fish stocks and seabirds, however, are often poorly correlated with the most widely used index of large scale climate variability in the region, the Pacific Decadal Oscillation (PDO). Here we define a new index of climate change, the North Pacific Gyres Oscillation (NPGO) and show that it is significantly correlated with previously unexplained fluctuations of salinity, nutrients and chlorophyll. Fluctuations in the NPGO are teleconnected from the tropics and forced by regional and basin-scale variations in wind-driven upwelling and horizontal advection - the fundamental processes controlling salinity and nutrient concentrations. Nutrient fluctuations drive concomitant changes in phytoplankton concentrations, and may force similar variability in higher trophic levels. The NPGO thus provides a strong indicator of fluctuations in the mechanisms driving planktonic ecosystem dynamics. Observations and global warming simulations show the NPGO amplitude to be increasing over the last 50 years; we expect the dynamics underlying the NPGO to play increasingly dominant roles in forcing decadal changes in marine ecosystems. http://iManu.Org/npgo
GC54B-05
Natural and Anthropogenically forced climate change in the Tropics.
We describe results from simulations using two general circulation models (HadMC3 and ECHO-G) which are driven by changes in solar irradiance and volcanic aerosol ("Natural forcings") and with CO2 and other greenhouse gases set to pre-industrial values. We examine the importance of naturally forced variability relative to internal variability. In both models we find that, especially in the tropics outside the Pacific, natural forcings significantly enhance temperature variability on decadal time-scales. Using another simulation of HadCM3, in which both human and natural forcings were included, we find that there is a significant simulated anthropogenic effect on the tropics as early as the mid-19th century. We also find evidence of a significant change in the flow of the River Orinoco which we speculate could be due to a northward shift of the tropical rain due to aerosols. If our results are applicable to the real world then they suggest that natural forcings needs to be considered an important mechanism for tropical variability and that humans may have influenced tropical climate prior to the 20th century. Finally we suggest that tropics may be a good place to look for paleo-records to examine the responses to external forcings and thus provide strong tests of climate models used to predict future climate change.
GC54B-06
COMPARING MECHNISMS OF SUBTROPICAL DRYING IN CLIMATE-CHANGE PROJECTION WITH THOSE CONTRIBUTING TO HISTORICAL DROUGHTS
Climate models display considerable agreement in their projection of future drying in subtropical regions, the Southwest US included, under common emission scenarios. The dynamical mechanisms leading to future droughts are a combination of thermodynamic effects related to the increase moisture carrying capacity of the atmosphere and a circulation change consistent of a broadening of the Hadley Cells, a related poleward shift in the location of the subtropical jets and a warming and increases static stability of the tropical troposphere. These mechanisms are distinctly different from the one responsible for droughts in the 19th and 20th century and, to the best of our knowledge, to the megadroughts that plagued the Southwest US in the medieval period. In the latter, a ?La Ni?a like? state in the tropical Pacific, with cold SST in the eastern equatorial Pacific and a cold tropical troposphere, lead to a similar, poleward displacement of the subtropical jet. We assert that only models that can simulate both kinds of mechanisms can be trusted in future climate projections. We also assert that it is important to identify which of these mechanisms dominate in the climate at any given time for reasons of attribution and prediction. In this presentation we contrast these two types of drought as they affect the Southwest US and the global circulation and attempt to determine which of these types dominates the present.
GC54B-07 INVITED
Vertical Mixing and Air-Sea 14CO2 Exchange in the Subtropics
We present a reconstruction of sub-tropical North Pacific surface water DELTA14C as recorded in a reef-building hermatypic coral from near Keahoe Bay on the west side of the Big Island of Hawai'i. Between 1946 and 1992 sub-annually resolved samples document a dynamic range of 232‰. Mean annual pre-bomb values are -55‰ and a post-bomb maximum of 160‰ is observed in 1971. To explore the sensitivity of vertical mixing parameterization schemes on subtropical air-sea CO2 exchange and the redistribution of anthropogenic CO2 as reflected by the time-history of oceanic 14C, these and similar data from additional subtropical locations (Bermuda, Rarotonga) are compared to DELTA14C simulated in a version of the Geophysical Fluid Dynamics Laboratory (GFDL) ocean general circulation model. In the sensitivity tests presented here, we were surprised to not find an adequate representation of the subtropical surface ocean 14C history and mixed layer depths, or at the very least a clear "better" choice. This "generic" ocean model contains widely used parameterizations of ocean mixing and is unable to successfully predict the surface DELTA14C time-history. These results imply a physical flaw or bias in at the very least this OGCM and likely others that use similar numerical mixing schemes.
GC54B-08
Strong tropical winds and upper ocean heat content
Vertical mixing in the ocean upper layer is strongly affected by intense winds. In turns, upper ocean heat content affects the evolution of tropical cyclones. We present results from a modeling study of the possible feedback between upper ocean heat content and hurricanes. The model indicates that for given atmospheric thermodynamical conditions, regimes characterized by intense (with deep mixing and large upper ocean heat content) and by weak (with shallow mixing and small heat content) tropical cyclone activity can be sustained. The feedback also amplifies the sensitivity of modeled cyclone power to atmospheric thermodynamic conditions. Observational data are discussed in light of the described interaction.
GC54B-09 INVITED
Are Patterns in Paleo-Hurricane Landfalls Significant? Statistical Comparisons with Modeled Hurricane Climatology
Coastal overwash deposits preserved within backbarrier sediments extend the documented record of tropical cyclone strikes back several millennia, providing valuable new data that help to elucidate links between tropical cyclone activity and climate variability. Certain caveats should be considered, however, when assessing trends observed within these paleo-storm records. For instance, gaps in overwash activity at a particular site could simply be artifacts produced by the random nature of these episodic events. Recently, a 5000 year record of intense hurricane strikes has been developed using coarse-grained overwash deposits from Laguna Playa Grande (LPG), a coastal lagoon located on the island of Vieques, Puerto Rico. The LPG record exhibits periods of frequent and infrequent hurricane-induced overwash activity spanning many centuries. These trends are consistent with overwash reconstructions from western Long Island, NY, and have been linked in part to variability in the El Niño/Southern Oscillation and the West African monsoon. Here we assess the statistical significance for active and inactive periods at LPG by creating thousands of synthetic overwash records for the site using storm tracks generated by a coupled ocean-atmosphere hurricane model set to mimic modern climatology. Results show that periods of infrequent overwash activity at the LPG site between 3600 and 1500 yrs BP and 1000 and 250 yrs BP are extremely unlikely to occur under modern climate conditions (above 99 percent confidence). This suggests that the variability observed in the Vieques record is consistent with changing climatic boundary conditions. Overwash frequency is greatest over the last 300 years, with 2 to 3 deposits/century compared to 0.6 deposits/century for earlier active regimes from 2500 to 1000 yrs BP and 5000 to 3600 yrs BP. While this may reflect an unprecedented level of activity over the last 5000 years, it may also in part be due to an undercounting of events in older sediments. Accounting for the 75 % lower accumulation rates in older sediments is alone not enough to explain the increased frequency of event deposits observed in the historic record. However, the most recent active interval is only 300 yrs. The variance in frequency over this time period is relatively high (2σ = 1.4 deposits/yr) and demonstrates the limitations associated with estimating reoccurrence intervals for extreme flooding using sediments from a single location.
GC54B-10
Impact of volcanic forcing on tropical temperatures over the last four centuries
Current knowledge of the impact of volcanism and other radiative forcings on the past climate of the tropics is limited, yet critically important for understanding abrupt climatic change over this region and the globe. Here we combine 19 high-resolution, well-dated records from tree rings, corals and ice cores into an annual composite time series that provides a view of volcanism's impact on tropical sea surface temperatures (SSTs) during the past four centuries. We find an association between explosive tropical volcanic eruptions and cold reconstructed temperatures, although the cooling is spatially variable across the tropics. Severe conditions following the unknown and Tambora, Indonesia eruptions of the early 1800s indicate that this was the coldest sustained period of the Little Ice Age in the tropics. The tropical impact of the 1600 Huaynaputina, Peru event appears much weaker than at higher latitudes, but the number of tropical proxies for this period is very low. The most severely cold reconstructed year is 1731 and may be related to several eruptions at this time, or to an unknown event. Our results have important implications for understanding the sensitivity of the tropics to natural and anthropogenic forcing.